With devices in the area of entertainment electronics in general and radio receivers in particular, it is frequently expedient or even necessary to acknowledge each manual control operation by the user with a short indicator tone. This applies particularly to compact radio receivers intended to be built into motor vehicles. One must assume that such devices must be frequently operated by the driver when driving. During the control operation the driver can direct only a small part of his attention to the control operation. He must continue to keep his vehicle under control. So he is often unable for example to look closely at a frequency display indicating whether a desired station finding has been performed successfully. If the radio receiver is equipped with an apparatus producing an indicator tone as soon as a control operation has been performed, for example the pressing of a button in the control panel, it is no longer absolutely necessary for the driver to direct his eyes to the display apparatus of the receiver.
With compact radio receivers as are used in particular for equipping motor vehicles, a trend has been apparent for some time to reduce costs during production by putting the entire electric circuit of the receiver together from very few large-scale integrated electronic components.
FIG. 4 shows a block diagram of an exemplary audio receiver. High frequency signals delivered by radio frequency antenna 10 are supplied to radio frequency processor RFP. Radio frequency processor RFP comprises high frequency preamplifiers, tuners, intermediate frequency amplifiers and filters as well as demodulators. A low frequency signal constituting the wanted signal from the received transmitter is supplied to audio signal processor ASP. This low frequency signal includes not only the demodulated audio signal but also auxiliary signals for stereo reception, ARI identification, RDS, etc., and others.
Audio signal processor ASP converts in particular the low frequency signal delivered by radio frequency processor RFP into audio signals which are supplied to audio power processor APP. Audio power processor APP includes in particular power amplifier 12 which amplifies the output signals delivered by audio signal processor ASP until they can be reproduced by loudspeakers 13. Further, audio power processor APP includes voltage regulator module 14 for power supply.
Other audio signals, for example from cassette recorders, compact disk players or the like, can be fed to audio signal processor ASP via further analog inputs.
The described audio receiver is controlled as a whole by microcontroller 15 connected with control unit 20 accessible to the user. The data exchange between microcontroller 15, on the one hand, and radio frequency processor RFP, audio signal processor ASP and audio power processor APP, on the other hand, takes place through a conventional inter-IC-bus.
FIG. 5 shows a schematic block diagram of audio signal processor ASP of the audio receiver shown in FIG. 4. Audio signal processor ASP has a number of analog audio signal inputs 50a to 50j connected with corresponding input lines of analog multiplexer 55. A stereo output signal from analog multiplexer 55 is fed via output lines 57a and 57b to a chain of series connected signal treating apparatuses 60, 62, 64, 66, 68.
First the audio signals pass into mute switching filter 60, then successively into volume control 62, second mute switching configuration with a soft mute switching property 64, bass control 66 and treble control 68. The stereo audio signals are then fed to a number of audio drive amplifiers 70 where they are conditioned until they are suitable for the drive of the power output stages in power amplifier 12 within audio power processor APP.
Each channel of the stereo signal is thereby divided into a first subchannel for a first loudspeaker to be mounted at the front of the passenger cubicle and a second subchannel for a second loudspeaker to be mounted at the back of the passenger compartment.
Further, the audio signal processor has a number of usual functional blocks 72 to 82 necessary for processing a stereo difference signal, an ARI signal, etc. In particular, a phase-locked loop (PLL) oscillator produces a 456 kHz signal that is phase locked with a stereo pilot signal as is known in the art. Finally, pause switching element 90 and power supply device 95 are provided.
In order to permit an indicator tone to be produced when a control operation is performed on control unit 20 of microcontroller 15, the audio receiver known from FIG. 5 has square wave signal oscillator 96 connected to analog signal input 50b of analog multiplexer 55 of audio signal processor ASP via signal shaper 97. Signal oscillator 96 can be designed for example as an a stable trigger element. However it is also possible to acquire a suitable square wave signal directly through microcontroller 15 if a program loop is executed which switches an output port (not shown) of microcontroller 15 connected with the input of signal shaper 97 alternatingly from a low level to a high level and vice versa with a predetermined time delay.
Since the reproduction of a straight square wave signal as an indicator tone in the loudspeakers leads to an undesirable and annoying sound, it is desirable to reduce the overtone content of the signal by signal shaper 97. Signal shaper 97 filters the square wave signal delivered by the signal oscillator so as to yield approximately a sinusoidal signal which is fed to analog multiplexer 55.
Signal oscillator 96 has enable input 98. When enable input 98 is held at a first electric level, signal oscillator 96 is blocked. When enable input 88 is put at a second electric level, signal oscillator 96 produces the square wave signal and an indicator signal becomes audible in loudspeaker 13 as soon as the corresponding input on analog multiplexer MUX is activated.
To permit signal shaper 97 to filter out the overtones of the square wave signal produced by signal oscillator 96 with sufficient quality without excessive damping of the base frequency, signal shaper 97 is dimensioned for a predetermined base frequency of signal oscillator 96 in the prior art.
This leads to the problem that only one preselected pitch of the indicator tone is possible in the known radio receiver shown in FIGS. 4 and 5. It would be desirable to be able to produce a multiplicity of different sounding indicator tones in an improved receiver in order to distinguish, for example, a successful control operation from one which did not cause the device to react in the way expected by the user (acoustic error message).
It is clear that audio signal processor ASP shown in FIGS. 4 and 5 is not the only possible embodiment of such a processor. In particular the audio signal processor can also be implemented without the functional units for treating FM stereo signals. The kind and number of audio frequency stages for influencing the audio signal can also vary in individual cases.
Even using only one indicator tone, the prior art circuit configuration of FIG. 5 has the disadvantage that it necessitates additional external components outside audio signal processor ASP preferably executed as an integrated circuit. When audio signal processor ASP is executed as an integrated circuit, another disadvantage is that a further signal input and thus a further package pin are necessary.